This invention relates to static convergence of cathode ray tubes for color television receivers.
Color display systems such as utilized in color television receivers include a cathode ray tube in which three electron beams are modulated by color-representative video signals. The beams impinge on respective color phosphor areas on the inside of the tube viewing screen. To accurately reproduce a color scene, the three beams must be substantially converged at the screen at all points on the raster. The beams may be converged at points away from the center of the raster by utilizing dynamic convergence methods or self-converging techniques, or a combination of both. Regardless of the methods utilized to achieve convergence while the beams are deflected, some provision must be made to statically converge the undeflected beams at the center of the screen. Static convergence devices are necessary because the effect of tolerances in the manufacture of electron beam guns and their assembly into the cathode ray tube neck frequently results in a misconverged condition.
Many static convergence devices include structure for producing adjustable magnetic fields. The devices are placed over the neck of the cathode ray tube and the magnetic fields are appropriately adjusted to provide for static convergence of the electron beams. Such adjustment is accomplished by moving magnetic field producing elements, by rotating magnetized rings about the cathode ray tube neck, or by rotating cylindrical magnets about an axis.
Other static convergence devices, such as disclosed in German Provisional Patent No. 2,611,633, filed Mar. 19, 1976, published Oct. 21, 1976, by Piet Gerard Joseph Barten et al., produce permanent nonadjustable magnetic fields. An auxiliary device having eight coils circumferentially located is placed around the cathode ray tube neck. Appropriately valued DC currents flowing through the coils establish a magnetic field which statically coverges the electron beams. The values of the DC currents provide data to a magnetizing apparatus which magnetizes regions within a sheath of magnetic material producing the aforementioned permanent nonadjustable magnetic fields. The magnetized sheath when placed over the neck of the cathode ray tube statically converges the electron beams.
It is desirable, when using such a magnetic sheath for static convergence, to eliminate the step of utilizing an auxiliary device for determining the locations within the magnetic sheath where magnetized regions are to be established.
A magnetizing apparatus, not utilizing such an auxiliary device, should have magnetizing areas arranged to facilitate uncomplicated operation when directly performing static convergence operations. Furthermore, to prevent adverse interaction of one magnetized region with another, the arrangement of magnetizing elements of the magnetizing apparatus should produce discrete magnetized regions in the sheath with no overlapping of magnetized areas.